Add CODEOWNERS (#2055)

Goal is to have DABs core team automatically added as reviewers so that
you don't have to click manually.

Based on this example:
https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners#example-of-a-codeowners-file

.github/CODEOWNERS

@@ -0,0 +1 @@
+* @pietern @andrewnester @shreyas-goenka @denik